lower tyre pressure in snow?

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Postby martine » Sat Dec 18, 2010 11:29 pm


I heard a 'tip' that if you're stuck in snow, letting your tyres down a little helps. Conversely my understanding was low-profile tyres are not so good in snow.

Are these 2 statements contradictory or is there something else going on?
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Postby Gareth » Sun Dec 19, 2010 5:43 am


I saw a Top Gear in Iceland where this was done but then they tyres were huge on large SUVs. For normal cars I've read it may help to get you out of a stuck situation but overall it won't be of much general benefit and would probably work against you. If it helps, some car manufacturers advise using slightly increased pressures for winter tyres - a number from VAG have handbooks that say add an extra 0.2 bar. Finally it is possible to buy winter tyres in a range of low profile sizes although the range is restricted when compared to summer tyres. Do you detect a common theme? Note that when I say it is possible I actually mean you've missed the boat the year.
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Postby jont » Sun Dec 19, 2010 10:25 am


ISTR when I did grass autotesting (ie a very low grip surface) that everyone would run much lower tyre pressures than usual to aid grip (a tyre that was normally 30psi might get dropped to 18). The downside of exuberant driving with low pressures is the risk of ripping the tyre off the rim :shock:

On the road it might get you out of a sticky situation, but if you've not got a pump then I wouldn't want to run any distance on that low a pressure.
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Postby chriskay » Sun Dec 19, 2010 11:06 am


Many years ago, when my wife was teaching in a town 10 miles away, in the snow she used to use our Honda 125 trail bike with 6psi in the tyres. Worked well.
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Postby Garrison » Sun Dec 19, 2010 7:06 pm


I only lower the pressure (to 15 psi) to get me out of trouble. I carry a pump on-board and I pump it back up as soon as I get on to better roads.
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Postby Astraist » Sun Dec 19, 2010 7:42 pm


Lowering the pressure is used in very cold conditions (=the air inside the tyre becomes more dense so the practical pressure remains identical) or on soft surfaces, where the convex shape of the underinflated tire captures gravel/fresh snow inside it to increase grip.

N.B. I would not consider a grassy surface to be "very low grip" in comparison to snow. The existance of vegetation like grass indicates that there are roots that make the base of the soil more rich and strong, giving the driver more grip.
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Postby waremark » Sun Dec 19, 2010 9:15 pm


Astraist wrote:N.B. I would not consider a grassy surface to be "very low grip" in comparison to snow.

Wet grass has extremely low grip. Have you ever met wet grass?
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Postby martine » Sun Dec 19, 2010 11:30 pm


Astraist wrote:...Lowering the pressure is used in very cold conditions (=the air inside the tyre becomes more dense so the practical pressure remains identical)...

What? No as the temperature reduces the pressure reduces...don't understand what you mean by 'practical pressure'.

So reducing the pressure may get allow me to get going in slippery conditions...but am I wrong in my understanding that narrow tyres are generally better for snow? If I'm not wrong why does reducing the pressure help?
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Postby Gareth » Mon Dec 20, 2010 9:33 am


martine wrote:am I wrong in my understanding that narrow tyres are generally better for snow?

I also understand this to be the case, hence my winter tyre choice. People talk about narrower tyres biting into the snow better but I think they may only be describing the effect. I wonder if part of the reason is due to the shape of the contact patch. Wider tyres have a wider but shorter contact patch, which is probably better for cornering when the conditions are good. Narrower tyres have a narrower but longer contact patch which possibly is more resistant to turning, and which might help in some way in slippery conditions.

I imagine StressedDave or others may have more knowledge about this.

martine wrote:If I'm not wrong why does reducing the pressure help?

Possibly because it allows the tyre to flex more than it was designed to do. It would also tend to elongate the contact patch, but if the tyre is then operating outside its design parameters there may be risk of damage, and certainly as jont mentioned, the chances of it coming off the wheel when cornering are increased.
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Postby martine » Mon Dec 20, 2010 10:09 am


Thanks Gareth.

Oh 'stressed' one - wherefore art though?
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Postby WS » Mon Dec 20, 2010 11:02 am


Gareth wrote:
martine wrote:am I wrong in my understanding that narrow tyres are generally better for snow?

I also understand this to be the case, hence my winter tyre choice. People talk about narrower tyres biting into the snow better but I think they may only be describing the effect. I wonder if part of the reason is due to the shape of the contact patch. Wider tyres have a wider but shorter contact patch, which is probably better for cornering when the conditions are good. Narrower tyres have a narrower but longer contact patch which possibly is more resistant to turning, and which might help in some way in slippery conditions.


Narrower tyres have better grip in snow because their smaller footprint means that the car's weight is transferred onto a smaller area than in wide tyres. Such tyres cut into snow more than wide tyres would.

I think it is important to understand that narrow tyres will give you more grip in snow only. On dry or wet tarmac they will probably increase braking distances and deteriorate cornering ability of the car because of the smaller footprint. So there is a tradeoff - if you want to be safer in the snow you need to remember you are less safe in no-snow conditions.
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Postby gfoot » Mon Dec 20, 2010 11:04 am


I imagine wider tyres (similarly, lower pressure tyres) would have a larger contact patch and would tend to rest on top of the snow. Narrower tyres would cut through and bite on whatever's underneath. But if what's underneath is ice, or compacted snow, that might not be good, and you might be better off floating on top of the snow with underinflated tyres.

Getting stuck and spinning a wheel probably results in very low traction under the wheel as the snow is compacted, but then deflating the tyre allows it to bite on fresher snow and hopefully make some progress.
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Postby Gareth » Mon Dec 20, 2010 11:33 am


WS wrote:Narrower tyres have better grip in snow because their smaller footprint means that the car's weight is transferred onto a smaller area than in wide tyres.

I'm not sure that's true as I understand the overall size of the contact patch is about the same when wheels with a narrower tread are used, because it's primarily a function of the weight of the car.

I'm certain StressedDave can clarify this point ...
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Postby Astraist » Mon Dec 20, 2010 12:00 pm


First, narrow tyres (or, alternativelly, overinflating a given tyre) can increase grip in fresh snow/frost and other conditions, where the tyre can penetrate the snow/mud/mould or other slippery surface and reach grippy tarmac beneath. If the snow is hardpacked, think or if there is ice underneath -- a wider tyre is better.

Second, about the coefficient of friction on these different surfaces: I believe there is a value for knowing the grip levels of various surfaces and knowing to identify them in the distance as you "read" the road surface. If the grass is wet, slippery mud might be created beneath it. Depending on the precentage of water and the type of soil, mud can supply grip levels similar to snow. However, a grassy surace by itself, grips better than the same soil without grass. Also, soil in general is slippery: More slippery than a wet road, for instance. But it's not like snow, not to mention ice.

It's also important to understand how changes in temperature change the air pressure inside the tyre. If, on a dry, sunny day, you drive you to a remote gas station to inflate your tyres, you will get there with hot tires. Heat makes air (and other substances) expand, so a standard air pressure of, say, 30 PSI -- would appear as 34 PSI (and that will be the pressure we would have to set the pump to). In the freezing cold, the same 30PSI might appear 25, and that would be the pressure we would have to fill, but in all of those cases -- the tyre itself is not changed: The net result is the pressure inside the tyre remains identical/
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Postby martine » Mon Dec 20, 2010 12:31 pm


Astraist wrote:...but in all of those cases -- the tyre itself is not changed: The net result is the pressure inside the tyre remains identical/

No...you ARE measuring the pressure in the tyre...it does reduce or increase depending on temperature. What else could the pressure gauge be showing?
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